Nitrogen control in argon oxygen refining of molten metal

ABSTRACT

The nitrogen content of molten metal may be controlled during refining by the selective subsurface injection of nitrogen during argon-oxygen decarburization. Nitrogen may be used to replace the argon entirely during the early stages of decarburization, and partially in the latter stages. In addition, nitrogen injection may be used after decarburization has been completed to achieve a final upward adjustment in the nitrogen conent of the metal.

United States Patent 1 Saccomano et al.

[ Aug. 28, 1973 NITROGEN CONTROL IN ARGON-OXYGEN REFINING OF MOLTEN METAL [75] Inventors: Joseph Michael Saccomano; John Douglas Ellis, Jr., both of Ft. Wayne, lnd.; Richard Jay Choulet, Phoenix, Ariz.; Frank Stuart Death, Carmel,

[73] Assignee: Joslyn Mtg. and Supply Co., I

Chicago, 111.; Union Carbide Corporation, New York, N.Y.

[22] Filed: Apr. 20, 1972 [21] Appl. No.: 245,733

Altland 75/60 3,597,191 8/1971- 3,046,107 7/1962 Nelson 75/60 3,666,439 5/1972 Ramachandran 75/130.5 2,855,293 10/1958 Savard 75/60 2,864,689 12/1958 Perrin 75/59 3,252,790 5/1966 Krivsky... 75/60 2,537,103v l/l951 Tancyn 75/l30.5

FOREIGN PATENTS OR APPLlCATlONS 691,124 5/1953 Great Britain 75/60 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney-Paul A. Rose et a1.

[57] ABSTRACT The nitrogen content of molten metal may be controlled during refining by the selective subsurface injection of nitrogen during argon-oxygen decarburization. Nitrogen may be used to replace the argon entirely during the early stages of decarburization, and partially in the latter stages. in addition, nitrogen injection may be used after decarburization has been completed to achieve a final upward adjustment in the nitrogen conent of the metal.

25 Claims, 3 Drawing Figures .REDAJfi'gION DECARBURIZATION STEP --LFINISHING P\:#[I\IITROGEN STEP I LLOYING I 1 1 a g l a I i l I *4 i i l Oz+Ar i Ar l 1 J Charge T/ME p PATENTED MIR 28 I915 SHEEIIBFS L u t m N VTI v2.6a

PATENIEBmcza I875 SKUBUF3 ZOFODQwK PATENTED AUG 28 825 SEEISBFS 0Z4 ZOE-9 5mm WSQK '1 NITROGEN CONTROL IN ARGON-OXYGEN REFINING OF MOLTEN METAL BACKGROUND This invention relates, in general, to methods for controlling the nitrogen content of molten metal by the selective injection of nitrogen gas during at least a portion of the time the metal is being refined by subsurface injection of oxygen and/or an inert gas. More specifically, the present invention relates to an improvement in the argon-oxygen decarburization of steels.

As used throughout the present specification and claims, the terms argon-oxygen refining or argonoxygen decarburization (AOD for short) refer to a process for refining molten metal by the subsurface injection of oxygen and argon or an equivalent inert gas, primarily for the purpose of lowering the carbon content of the melt. The basic AOD process is disclosed in Krivsky, U.S. Pat. No. 3,252,790 and an improvement thereon relating to programmed blowing is disclosed in Nelson et al., U.S. Pat. No. 3,046,107.

The AOD process is a duplex process,v particularly useful for refining of stainless steels without substantial loss of chromium, by using a furnace, commonly an arc-furnace, for melting scrap and alloy. After the molten metal is deslagged, it is transferred to a refining vessel in which it is decarburized by subsurface blowing with an inert gas-oxygen mixture (in commercial practice an argon-oxygen mixture), reduced, finished and tapped intoa teeming ladle. Finishing may include deslagging, desulfurization and addition of alloying elements to obtain final adjustment in the in the refining vessel before tapping. A suitable refining vessel, which can be rotated to the horizontal position for charging, holding, sampling and tapping, is disclosed by Saccomano and Ellis in French Patent No. 2,056,845, which corresponds to U.S. Pat. No. 3,703,279. During refining, the vessel is rotated to the vertical position and varying ratios of argon and oxygen mixtures are injected through tuyeres mounted in the bottom or sides of the vessel.

Historically, steelmakers have had little control over the final nitrogen content of the steels they produced, since the nitrogen content of the steel has generally been a characteristic of the particular process employed for making it. In carbon steels, for example, the residual nitrogen levels of steels made by the open hearth process have been the lowest, with progressively higher nitrogen levels being obtained in steels made by the BOP process, the electric furnace process and those made by the Bessemer process. In fact, the high residual nitrogen levels of Bessemer steels have prevented the use of such material in certain major applications, and have consequently contributed to the virtual abandonment of this process in the United States and to severe curtailment of its use in other parts of the world.

In stainless steelmaking, most of the worlds production is made in electric arc-furnaces. With this process, residual nitrogen levels depend upon a number of variables; the most important of which include the melting rate, the type of charge, the melt composition, the refining time and the finishing practice. Because these variables are generally fixed by over-riding consider ations, such as economics, the availability of raw materials, alloy specifications and other residuals such as oxygen and sulfur, the steelmaker has had little practical control over the residual nitrogen level. It is, of course,

possible to increase the final nitrogen level of a melt by adding nitrogen-bearing electrolytic manganese or ferroalloys to it, but this procedure has several disadvantages. In the first place, the manganese and ferroalloys are expensive; and secondly, control of the nitrogen level in the melt is often erratic. If the quantity of the allow addition is large, the melting and dissolution of the added alloy absorbs large amounts of heat, requiring additional furnace time for completion of the heat. Hence, nitrogen added in this way, is added at the expense of productivity and economy of operation.

Although the Krivsky patent, referred to above, teaches that nitrogen is equivalent to argon as an inert gas in the AOD process, this equivalence is applicable only insofar as its function during the decarburization step is concerned, and even then only to those grades of steel in which large residual amounts of nitrogen may be tolerated. This is so because the substitution of nitrogen for argon will result in a refined melt containing an amount of nitrogen which is close to the quantity of nitrogen dissolved in the melt which is in equilibrium with a nitrogen containing atmosphere surrounding the melt at ambient pressure and at the temperature and composition of the melt. This quantity is commonly referred to as the equilibrium nitrogen level, and may be calculated from theoretical thermodynamic considerations by techniques known to those skilled in the art; see: for example, Chipman and Corrigan Prediction of the Solubility of Nitrogen in Molten Steel, Trans. AIME. Vol. 233, July I965.

OBJECTS It is an object of this invention to provide a process which produces metal containing dissolved nitrogen of any desired level from about 10 ppm to the equilibrium level of nitrogen in the molten metal by a process which is both simple to perform and economical to operate.

It is another object of this invention to provide an improvement on the AOD process which is more economical to operate, by replacing during decarburization, one or more selected portions of the argon by less expensive nitrogen without exceeding the nitrogen specification for the metal produced.

It is a further object of this invention to provide a process in which final nitrogen adjustment is made just prior to tapping of the heat, without substantially effecting the temperature of the heat and without regard to the initial nitrogen content of the heat.

SUMMARY OF THE INVENTION The above and other objects which will readily be apparent to those skilled in the art, are achieved by the present invention one embodiment of which comprises: in a process for refining molten metal comprising the step of decarburizing a mass of molten metal by injecting into said mass from beneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon, zenon and nitrogen, the improvement comprising: producing a refined molten metal mass having a predetermined nitrogen content within the range of from about I0 ppm to about percent of the equilibrium level, by (a) injecting a gas mixture consisting essentially of oxygen and nitrogen throughout a first period of the decarburization step, wherein the percentage of nitrogen in said gas mixture is maintained such that the partial pressure of the nitrogen in the ambient atmosphere in contact with the melt is greater than the partial pressure of nitrogen in equilibrium with the predetermined nitrogen content sought for the refined melt, thereby causing the nitro gen content of the melt at the end of said first period of said decarburization step to be greater than the predetermined nitrogen content sought for the refined melt, and thereafter (b) substituting an inert gas other than nitrogen in place of said nitrogen in said gas'mixture during the remainder of said decarburization step, and continuing the injection of said other inert gas until the nitrogen content of the melt is reduced to said predetermined value.

In conventional steelmaking practice, the decarburization step is generally followed by a reduction step. Thereafter, the melt may be finished by one or more of pan the optional steps of deslagging, desulfurizing, deoxidizing, adjusting the temperature and adjustment of the composition of the melt by the addition of alloying materials.

According to an alternate embodimentof the invention, a three component gas mixture containing oxygen, nitrogen and another of said inert gases (preferably argon) may be injected during at least a portion of the decarburization step. That is, the three component gas mixture may be used during only the first period of the decarburization step, during only the second or subsequent periods of the decarburization step, or during the entire decarburization step. If the three component gas mixture is used during the entire decarburization step or during the second or subsequent periods thereof, then in order to obtain the specific nitrogen level sought in the refined melt, the percentage of nitrogen in the three component gas mixture is maintained such that the partial pressure of the nitrogen in the gaseous atmosphere in contact with the melt is equal to the calculated partial pressure of nitrogen divided by X, where X 0.7 to 1.0, in equilibrium with the predetermined 'nitrogen level sought in the refined metal melt.

Following decarburization of the melt, which may be carried out in accordance with any of the embodiments of the present invention described above, or in accordance with conventional AOD practice, nitrogen gas or a mixture of nitrogen and argon may be injected into the melt for a sufficient time to increase the nitrogen content of the molten metal mass to any desired level of nitrogen up to within about 90 percent of its equilibrium level. Optionally, this alloying nitrogen injection may follow the finishing steps rather than the de carburization step.

The term molten metal as used throughout the present specification and claims is intended to include low carbon iron, carbon steels, stainless steels, ferrous alloys and nickel based alloys. These may contain chromium, tungsten, vanadium, zirconium, copper, aluminum, silicon, sulfur, titanium, manganese, molybdenum and other commonly used alloying ingredients. The term stainless steel is intended to include ferrous alloys containing about 13-40 percent chromium.

The term decarburization is used to mean lowering of the carbon content of the molten metal from any given level to any desired lower level by the injection of oxygen into the melt. The term mass is intended to mean a batch or heat of molten metal, as well as a changing mass as in a continuous process.

The term reduction is used to mean the recovery from the slag of metallic materials, such as, for example, chromium or manganese which were oxidized during the decarburization step, by adding a less valuable material such as silicon or aluminum which has a greater affinity for oxygen than the desired materials, thereby causing the reduced chromium or manganese metal to go backinto the melt. The reduction, however, is 'not limited to being carried out with solid materials. As a result of the efficiency with which dissolved hydrogen is removed from the melt with the inert gas, it is possible .to reduce the oxidized metallic materials in the slag by the injection of hydrogen yielding gases such as hydrogen, ammonia (NH,),- or a hydrocarbon gas, i.e., methane, propane or natural gas.

The term desulfurizationT is used to mean the lowering of the sulfur content of the melt by providing the proper thermodynamic and kinetic conditions to move the sulfur from the molten metal phase to the slag phase.

The term finishing is used to mean any or all of the conventional steps after reduction which prepare the molten metal for tapping and casting, e.g., deslagging, desulfurization, final composition adjustment, temperature adjustment and deoxidation.

THE DRAWINGS FIG. 1 is a graphic representation'of the change in the nitrogen content of a melt during refining in accordance with the present invention, in which an oxygennitrogen mixture is injected during a first period of the decarburization step, followed by a second period in which argon is either substituted for the nitrogen in the mixture (curve X) or added to the mixture (curve Y), and then finished with argon alone.

FIG. 2 is a graphic representation of the change in the nitrogen content of a molten metal heat refined in accordance with the present invention, in which a three component gas mixture (curve A) of oxygen, nitrogen and argon is used throughout the decarburization step, compared to two curves illustrating the use of oxygen in combination with either argon (curve B) or nitrogen (curve C) in accordance with the prior art.

FIG. 3 is a graphic representation illustrating the change in the nitrogen content of a melt during decarburization with a mixture of argon and oxygen and reduction and finishing with argon alone in accordance with the prior art, followed by nitrogen injection for alloying.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows that during the first period of the decarburization step, using a mixture of oxygen and nitrogen, the level of nitrogen in the melt will be raised close to its equilibrium level N, at the particularly conditions of melt temperature and composition, and ambient gas pressure in the refining vessel. During the second period of the decarburization step, the substitution of argon for the nitrogen (curve X) will cause a rapid decrease in the nitrogen level of the melt. This second period is continued until the desired carbon level and a specified nitrogen level N,, depending upon the final nitrogen level desired on tapping N, is reached. Since continued argon injection during the reduction and tinishing' steps will cause additional, though relatively small, lowering of the nitrogen level, it will be apparent that the point N, at which the switch is made from nitrogen to argon will depend on the amount of argon injected in both the second period of the decarburization step, as well as upon' the ultimate nitrogen level N sought in the tapped metal.

Curve Y discloses the path that the nitrogen level in the melt will take if a higher nitrogen level N is sought in the tapped heat and in which a three component argon-oxygen-nitrogen gas mixture is used during the second period of the decarburization step. Curve Z shows the path which would be taken by the nitrogen level in the melt if the same final nitrogen level N: were sought using a three component nitrogeb-oxygen-ar'gon mixture during the second period of the decarburization step. In this ca se the first period would stop sooner at time T rather than at time T as in the previous example, since the rate at which the nitrogen level in the melt drops is slower with nitrogen in the blowing mixture than without any nitrogen as 'in curve X.

In FIG. 2 curve C shows the increase in the nitrogen content of the melt using the prior art technique of oxygen-nitrogen decarburization, followed by nitrogen injection during the reduction and finishing steps. It will be noted that the nitrogen level continues to rise throughout, and that the level of nitrogen in the tapped heat will be close to its equilibrium level. Curve B discloses conventional argon-oxygen practice in which argon and oxygen are used throughout the decarburization step followed by argon injection during the reduction and finishing steps. It will be seen that the nitrogen level of the melt in curve B continues to decrease throughout the refining steps, with the result that an extremely low level of nitrogen N, is obtained in the tapped heat.

Curve A, which represents an example of the present invention, discloses a three component gas mixture of argon, nitrogen and oxygen which is injected throughout the decarburization step, followed by a two component argon-nitrogen mixture during reduction and finishing, in which the nitrogen level of the melt is gradually increased throughout the refining process. It will be evident that by proper ratioing of the quantity of the gases in the three component mixture, curve A may be caused to follow a path lying anywhere in the area on the graph between curve C and curve B. This is necessarily so, because in the limiting case, where the amount of argon in the gas mixture is zero, curve A is obtained; whereas in the limiting case where the amount of nitrogen in the gas mixture is zero, curve B is obtained. For example, the cruves X and Z from FIG. 1 may be superimposed onto FIG. 2 where they are shown as broken lines X and Z. The slope of the curve will be determined principally by the relative amount of nitrogen in the blowing mixture. Furthermore, it is possible by use of an appropriate three component gas mixture (the proportions of which will vary during the decarburization step) to arrive at a nitrogen level in the melt lying anywhere between the nitrogen values represented by points N and N of the graph. Thereafter, the nitrogen level may be increased to level N, by blowing with nitrogen, decreased to level N, by blowing with argon, or kept essentially the same at N by use of an appropriate argon-nitrogen mixture.

It has been observed in the commercial practice of the AOD process on stainless steel, in which argon is the sole inert gas used, that the final nitrogen levels obtained in the melt after decarburization, reduction and finishing are 30-50 percent lower than that normally obtained by conventional arc-fumace practice. On the other hand, as noted earlier, a significantly different problem has been found if nitrogen is used as the sole inert gas during the refining operations. In the latter case, by the end of the refining steps, the dissolved nitrogen in the melt approaches the equilibrium level. While this is not surprising based on theoretical considerations, previous practical experience has indicated that a close approach to the theoretically calculated equilibrium level is never reached in a practical system; see Ward, The Physical Chemistry of Iron and Steelmaking 1952, pages 182-183. Furthennore, in the Bessemer process, where the melt is blown with air (containing approximately 79 percent nitrogen) the observed nitrogen content of the decarburized steel is normally 0.01 to 0.02 percent, while the equilibrium level is about 0.04 percent. Thus, the measured final nitrogen content in such melts is typically 25-50 percent of the value predicted by theoretical equilibrium considerations. The'idea that nitrogen pickup does not take place in the melt under oxidizing conditions, such as are present during decarburization, is also taught by Tanczyn in US. Pat. No. 2,537,103..

In contrast to such prior art views, it has been found that in practicing the AOD process with nitrogen as the inert gas, much higher levels of nitrogen are left in the decarburized melt than would have been anticipated. For example, using nitrogen and oxygen in refining a 17 ton heat of type 304 stainless steel, the actual nitrogen content was 0.136 percent, whereas the equilibrium level is about 0.145 percent. Thus, the melt reached almost 94 percent'of the equilibrium level. Nitrogen injected during the reduction, desulfurization and finishing steps raise the level to 0.207 percent, compared to a calculated equilibrium value of 0.247 percent, i.e., about 80 percent of the equilibrium value.

FIG. 3 shows graphically the change in nitrogen level which may be achieved by nitrogen alloying, i.e. rapidly increasing the nitrogen level of the melt by the injection of nitrogen gas following decarburization with a mixture of argon and oxygen, and argon blowing during the reduction and finishing steps. The use of a pure nitrogen blow at the end of these steps may be used to quickly and conveniently raise the nigrogen level of the heat to any level up to close to within about percent of the equilibrium value of nitrogen at the conditions of the melt.

The following examples will serve to illustrate the different preferred embodiments of the present invention.

EXAMPLE 1 Example 1 below shows the results obtained on a 16 1% ton test heat in which a three component mixture of argon, oxygen and nitrogen was used throughout the decarburization step, followed by a reduction step using a mixture of argon and nitrogen. The type 304 stainless steel melt composition at the beginning of the decarburization step contained 0.17% C, 0.96% Mn, 0.27% Si, 19.38% Cr and 8.54% Ni. A maximum residual nitrogen level of 0.08 percent was sought in this heat. Table 1 below shows the temperature, flow rates of the gas and nitrogen content of the melt at the end of each noted period of the decarburization step and at the end of the reduction step. It should be noted that the decarburization was carried out in three steps, i.e., with three dilferent gas mixtures and oxygen to inert gas ratios. N.A. indicates the figure is not available.

TABLE 1 Temp. Ar N, N (F.) 1000 cfh) (wt.%) Start 2925 .037 Decarb. 1st per. 3075 19 5.5 1.5 .057 Decarb. 2nd per. 3100 7 12 10.5 1.5 .058 Decarb. 3rd per. 3110 9 14.5 1.5 .057 Reduction N.A. 9.0 1.0 .061

The equilibrium partial pressure of nitrogen in a steel melt of the composition of Example 1 containing 0.08 percent nitrogen is calculated in accordance with the techniques disclosed by Chipman and Corrigan, in the article referred to previously, to be approximately 0.1 atmospheres. Therefore, sufficient nitrogen was injected, along with the argon and oxygen, to provide .a nitrogen partial pressure in the atmosphere in contact with the surface of the melt of 0.1 atmospheres. During decarburization, considerable amount of CO care evolved which dilute the N partial pressure. Hence, larger ratios of nitrogen to argon can be used during the early portions of the decarburization, than during the latter portions and during reduction and finishing. During the latter steps, since no CO is evolved, the nitrogen to argon ratio must be lowered and controlled to give a nitrogen partial pressure of no more than 0.1 atmospheres. As can be seen from the above example, the final nitrogen level obtained was 0.061 percent as against a calculated equilibrium level of 0.08 percent, showing that the nitrogen level attained by the melt during the refining process was greater than 75 percent of the calculated equilibrium level.

It has been found that the results demonstrated by Example 1 above are highly reproducible, not only in the same vessel, but from one vessel to another, and that the nitrogen level reached after refining is consistently greater than 70 pereentofthe calculated equilibrium level; usually between 75-90 percent of the equilibrium level. In actual practice, the closeness with which the equilibrium level is approached depends upon the specific design of the vessel, the size and composition of the heat, the depth of the melt, the number and arrangementofinjection tuyeres, gas flow rates and other system variables. However, after one or two trials the appropriate equilibrium factor, which will lie between 0.7 and 1.0 for any particular vessel system, can be easily determined.

While Example 1 above represents one embodiment of the present invention, it has also been found possible by means of the present invention to significantly improve upon the conventional AOD process so as to make it more economical, by permitting substitution of much of the argon during the early stages of decarburization by less expensive nitrogen gas.

In many studies previously made on the absorptionclesorption kinetics of nitrogen, it has been found that degassing processes using either vacuum or argon are very ineffective with respect to nitrogen removal. This has been found, for example, by Pehlke and Elliott; see: Solubility of Nitrogen in Liquid lron Alloys I1 Kinetics, Trans. of Met. Soc AIME (1963) to be particularly so when surface active elements such as oxygen or sulfur are present. Consequently, it was expected that under oxidizing conditions, such as are present during argon-oxygen decarburization, it would be difficult to remove significant amounts of dissolved nitrogen from the melt. However, it has been unexpectedly found that even under the oxidizing conditions of the decarburization step, significant amounts of nitrogen can be removed. As a result, it is possible to substitute nitrogen for argon as the inert gas component during at least the early phases of the decarburization period, even though a considerable amount of nitrogen is absorbed by the melt. The amount of nitrogen which can be substituted for argon is related to the ultimate level desired in the melt at tapping. For example, with type 304 stainless steel, if the aimed for residual level of nitrogen is less than 0.05 percent, nitrogen can be used during the decarburization step until approximately percent of the oxygen calculated as necessary for decarburization has been injected. in general, the point at which nitrogen is replaced by argon takes place when between 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected. This quantity of oxygen is calculated by conventional stoichiometric means, taking intoaccount the oxygen necessary to oxidize not only the carbon to be removed as CO, but also to oxidize the silicon and other metallic elements in the melt which conventionally enter the slag as oxides. At such calculated point, argon is used to replace the nitrogen for purposes of further decarburization, as well as for lowering the dissolved nitrogen content of the melt to the desired level.

Curves X and Z in FIG. 1 illustrate schematically the change in the nitrogen content of a melt during the decarburization step. The point in time, T or T at which it is necessary to either switch from nitrogen to argon (curve X) or to reduce nitrogen and add argon (curve 2) is also a function of the overall blowing program, the melt composition and the refining temperature. Hence, it is necessary with each vessel system to run several trial heats from which, however, it is easy to establi h the switch point at which the changeover from nitrogen to argon should be made.

EXAMPLE 2 The following example illustrates the embodiment of the present invention in which nitrogen is used as the sole inert gas during the first period of the decarburization step, followed by the second period in which argon is used to replace the nitrogen. The type 303 stainless steel melt prior to decarburization contained: 0.78% C, 0.51% Mn, 0.41% Si 18.25% Cr and 8.05% Ni. The heat size was 17 tons. Table 2 below shows the changes in temperature, carbon content, gas flows and nitrogen level at the start, during the first and second periods of the decarburization step, and after the reduction step.

It can be seen from Table 2 above that the nitrogen level in the melt rose during the first period of the decarburization step from 0.042% to 0.075% as a result of blowing with a 2 to 1 ratio of oxygen to nitrogen. However, during the second period of the decarburi- EXAMPLE 3 Example 3 illustrates the use of nitrogen throughout the entire decarburization step, wherein; however, the ratio of :N was changed from 2:1 in the first period, to 1:2 during the second period, and demonstrates the high residual level of nitrogen in the melt at the end of the oxygen blowing step. Table 3 below shows the two stage decarburization step followed by reduction, desulfurization and finishing, as well as the duration of each of these steps. The change in the carbon level during the process, as well as the nitrogen level, together with the oxygen, argon and nitrogen gas flow rates are also shown.

TABLE 3 1,000 c.-f.h.

Temp. Time C N (\vt. F.) (min) (percent) 02 Ar N2 percent)- Start 2, 720 0 1. 10 041, Dccarb. 1st I per 3,120 35 0. 26 16 8 076:

Decarb. 2nd

pet ,160 19 0. 06 7 14 125 Reduction 3,070 4 .A. 14 -.094 Desulfurization 2, 910 4 N.A. 14 079 Finishing... 2, 810 2 0. 14 .073

As can be seen from Table 3 above, thehig h le'vl of nitrogen at the end of the decarburization step, 0.125% is lowered by subsequent argon blowing during reduction, desulfurization and finishing to 0.073%.

EXAMPLE 4 Example 4 below illustrates the embodiment of the present invention in which nitrogen is used as the inert gas during the first period of the decarburization step, followed by the second step in which argon is used to replace the nitrogen. The type 430 stainless steel melt prior to decarburization contained: 0.35% C, 0.34% Mn, 0.36% Si, 16.22% Cr and 0.14%Ni. The heat size was 17 tons. Table 4 below shows the changes in temperature, carbon content, gas flows and nitrogen level at the start, and after the first and second steps and the decarburization as well as after the reduction steps.

" Mani a 1,000 c.f.h.

(1 (111111.) (percent) 02 A1- Nz percent) {m1 3,080 40 0. 1] Hi H mi Reduction... 3, 040 4 0. 05 10 .056

Sample taken after minutes of decarburization.

As can be seen from Table 4 above, the high nitrogen level of 0.075% caused by use of nitrogen during the first period of decarburization was brought down to 0.056% by the argon blow. The 0.036% nitrogen level shown in the table was not an actual starting value (which was not taken) but rather is typical for the melt composition in question. It should be noted that oxygen flow was maintained on during reduction. This was not done for purposes of decarburization, but rather to prevent the temperature of the heat from falling excessively. A

EXAMPLES 5-8 During finishing, when the melt composition is adjusted to meet its ultimate specifications, nitrogen addition can be made simply by injecting nitrogen gas into the melt for a period of time depending on the final level desired. This procedure may be referred to nitrogen alloying. It has been found unexpectedly that the rate of nitrogen pick-up is highly reproducible, and for the reasons given before, unexpectedly high. By the use of this technique, it is possible to obtain residual nitrogen levels in the melt of up to about 50 percent of the equilibrium level at 1 atmosphere of N in a rapid and economic manner. Levels above 50 percent of equilibrium may also be obtained; however, the rate of N pick-up begins to decrease, and hence becomes less ef' ficient. Table-5 below shows five examples in which the nitrogen level was increased .substantially with only- 20-69 seconds of N blowing.

TABLE 5 Example Type -lnitial Final N, Time No. Steel N N (1000 (sec.)

' cfh) Although the examples given above have treated the various embodiment of the invention as separate individual procedures, it will be obvious to those skilled in the art that the different embodiments may be used in various combinations in order to achieve the maximum advantages of gas economics, reproducibility and nitrogen control in the ultimate product.

What is claimed is: percent of the arrangement of injection 1. In a process for refining molten metal comprising the step of decarburizing a mass of molten metal by injecting into said mass from underneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon, xenon and nitrogen, the improvement comprising:

producing a refined molten metal mass having a predetermined nitrogen content within the range of from about 10 ppm to about percent of the equilibrium level, by (a) injecting a gas mixture consisting essentially of oxygen and nitrogen throughout a first period of the decarburization step, wherein the percentage of nitrogen in said gas mixture is maintained such that the partial pressure of the nitrogen in the ambient atmosphere in contact with the melt is greater than the partial pressure of nitrogen in equilibrium with the predetermined nitrogen content sought for the refined melt, thereby causing the nitrogen content of the melt at the end of said first period of the decarburization step to be greater than the predetermined nitrogen content sought for the refined melt, and thereafter (b) substituting an inert gas other than nitrogen in place of said nitrogen in said gas mixture during the remainder of said decarburization step, and continuing the injection of said other inert gas until the nitrogen content of the melt is reduced to said predetermined value.

2. The process of claim 1 wherein said other inert gas is argon.

3. The process of claim 2 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.

4. The process of claim 2 wherein the first period of said decarburization is terminated after 5070 percent of the oxygen calculated as necessary for the decarburization step has been injected.

5. The process of claim 2 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.

6. The process of claim 2 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase. the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.

7. In a process for refining molten metal comprising the step of decarburizing a mass of molten metal by injecting into said mass from underneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon xenon and nitrogen, the improvement comprising:

producing a refined molten metal mass having a predetermined nitrogen content within the range of from about ppm to about 90 percent of the 'equilibrium level, by (a) injecting a gas mixture consisting essentially of oxygen and nitrogen throughout a first period of the decarburization step, wherein the percentage of nitrogen in said gas mixture is maintained such that the partial pressure of the nitrogen in the ambient atmosphere in contact with the melt is greater than the partial pressure of ni trogen in equilibrium with the predetermined nitrogen content sought for the refined melt, thereby causing the nitrogen content of the melt at the end of said first period of the decarburization step to be greater than the predetermined nitrogen content sought for the refined melt, and thereafter (b) adding an inert gas other than nitrogen to said oxygennitrogen gas mixture during the remainder of said decarburization step and continuing the injection of said other inert gas until the nitrogen content of the melt is reduced to said predetermined value.

8. The process of claim '7 wherein said other inert gas is argon.

9. The process of claim 8 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.

10. The process of claim 8 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.

11. The process of claim 8 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.

12. The process of claim 8 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.

13. In a process for refining molten metal comprising the step of decarburizing said molten metal by injecting into said molten metal from underneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon, xenon and nitrogen, the improvement comprising:

producing a refined molten metal charge having a predetermined nitrogen content within the range of from about 10 ppm to about percent of the equilibrium level, by injecting a three component gas mixture of oxygen, nitrogen and another of said inert gases, throughout at least a first period of the decarburization step, and wherein the percentage of nitrogen in the said mixture is maintained such that the partial pressure of the nitrogen in the gaseous atmosphere in contact with the melt is equal to the partial pressure of nitrogen in equilibrium with the predetermined nitrogen content sought in the refined melt divided by X where X; 0.7 to 1.0.

' 14. The process of claim 13 wherein said three component gas mixture is injected throughout the entire decarburizationstep.

15. The process of claim 14 wherein said other inert gas is argon.

, 16. The process of claim 15 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys. r

17. The process of claim 15 wherein the first period of saiddecarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.

18. The process of claim 15 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.

19. The process of claim 15 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.

20. The process of claim 13 wherein subsequent to termination of said first period of the decarburization step, the nitrogen flow is reduced to zero.

21. The process of claim 20 wherein said other inert gas is argon.

22. The process of claim 21 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.

23. The process of claim 21 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.

24. The process of claim 21 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.

25. The process of claim 21 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.

UNITED sums PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,754,894 Issue Date August 28, 1973 Inventor) R.J. Choulet et al.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected n shown below:

In col. 2, line 58, "zenon" should read xenon In col. 5, line 10, "nitrogeb" should read -nitrogen In col. 6, line 33, "raise" should read raised In col. 7, line 20 "amount" should read amounts In col. 7, line 20, "care" should read are In col. 7, line 38, "percentofthe" should read percent of the should read In col. 7, line 44, "arrangementofinjection arrangement of injection In col. 10, line 37, after "What is claimed is" delete percent of the arrangement of injection Signed and sealed this l7th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. M' SON Commissioner of Patents Arresting Officer and Trademarks 

2. The process of claim 1 wherein said other inert gas is argon.
 3. The process of claim 2 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.
 4. The process of claim 2 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.
 5. The process of claim 2 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.
 6. The process of claim 2 wherein the decarburizatiOn step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.
 7. In a process for refining molten metal comprising the step of decarburizing a mass of molten metal by injecting into said mass from underneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon xenon and nitrogen, the improvement comprising: producing a refined molten metal mass having a predetermined nitrogen content within the range of from about 10 ppm to about 90 percent of the equilibrium level, by (a) injecting a gas mixture consisting essentially of oxygen and nitrogen throughout a first period of the decarburization step, wherein the percentage of nitrogen in said gas mixture is maintained such that the partial pressure of the nitrogen in the ambient atmosphere in contact with the melt is greater than the partial pressure of nitrogen in equilibrium with the predetermined nitrogen content sought for the refined melt, thereby causing the nitrogen content of the melt at the end of said first period of the decarburization step to be greater than the predetermined nitrogen content sought for the refined melt, and thereafter (b) adding an inert gas other than nitrogen to said oxygen-nitrogen gas mixture during the remainder of said decarburization step and continuing the injection of said other inert gas until the nitrogen content of the melt is reduced to said predetermined value.
 8. The process of claim 7 wherein said other inert gas is argon.
 9. The process of claim 8 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.
 10. The process of claim 8 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.
 11. The process of claim 8 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.
 12. The process of claim 8 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.
 13. In a process for refining molten metal comprising the step of decarburizing said molten metal by injecting into said molten metal from underneath the surface thereof, oxygen and at least one inert gas selected from the group consisting of helium, neon, argon, xenon and nitrogen, the improvement comprising: producing a refined molten metal charge having a predetermined nitrogen content within the range of from about 10 ppm to about 90 percent of the equilibrium level, by injecting a three component gas mixture of oxygen, nitrogen and another of said inert gases, throughout at least a first period of the decarburization step, and wherein the percentage of nitrogen in the said mixture is maintained such that the partial pressure of the nitrogen in the gaseous atmosphere in contact with the melt is equal to the partial pressure of nitrogen in equilibrium with the predetermined nitrogen content sought in the refined melt divided by X where X 0.7 to 1.0.
 14. The process of claim 13 wherein said three component gas mixture is injected throughout the entire decarburization step.
 15. The process of claim 14 wherein said other inert gas is argon.
 16. The process of claim 15 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.
 17. The process of claim 15 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.
 18. The process of claim 15 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.
 19. The process of claim 15 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value.
 20. The process of claim 13 wherein subsequent to termination of said first period of the decarburization step, the nitrogen flow is reduced to zero.
 21. The process of claim 20 wherein said other inert gas is argon.
 22. The process of claim 21 wherein the molten metal is selected from the group consisting of carbon steel, stainless steel, ferrous alloys and nickel based alloys.
 23. The process of claim 21 wherein the first period of said decarburization is terminated after 50-70 percent of the oxygen calculated as necessary for the decarburization step has been injected.
 24. The process of claim 21 wherein following the decarburization step, nitrogen alone is injected into the melt for a sufficient time to increase the nitrogen content of the refined molten steel to any desired level up to within about 90 percent of its equilibrium value.
 25. The process of claim 21 wherein the decarburization step is followed by reduction and finishing steps, and wherein nitrogen alone is injected into the melt during or after the finishing steps to increase the nitrogen content of the refined melt to the desired level up to within about 90 percent of its equilibrium value. 